This invention relates to an optical disk recording device of a type which records information by projecting a laser beam on a recording surface of an optical disk to form pits thereon and, more particularly, to an optical disk recording device, which is capable of preventing occurrence of an adverse effect on a signal-to-noise ratio of a wobble signal when a recording is made by using a higher recording speed than a normal recording speed.
As a manner of recording information on a write-once type optical disk on the basis of the CD-WO (CD Write Once) standard, it has been proposed to record information by using not only a normal speed but also a higher speed than the normal speed such as a double speed and a quadruple speed. By using such high speed recording, for example a quadruple speed, if is possible to store data such as CD-ROM data and audio data in a hard disk, read the stored data in the form of quadruple compressed data, and record the read out data by driving an optical disk at a quadruple speed. In case a double speed is employed, for example, a Compact Disc player may be driven at a double speed and an optical disk may be driven at a double speed to copy reproduced data.
In a prior art optical disk recording device, it has been proposed to perform a high speed recording by increasing power (pit power) of recording laser beam in a pit forming section (i.e., pit period) as shown in FIG. 2 for accurately forming pits on the optical disk.
According to the CD-WO standard, a groove called "pregroove" is preformed on the recording surface of an optical disk and, during recording of information, recording is made by tracking this pregroove under a tracking control. This pregroove is not a straight line but is undulated with a particular frequency and this undulation is called "wobble". In the recording mode, this wobble is detected on the basis of a residual component of a tracking error signal and a CLV (constant linear velocity) spindle control is realized by PLL-controlling a disk motor so that the frequency of the detected wobble will become a predetermined frequency (e.g., 22.05 kHz in a normal recording speed mode).
If the device is designed in a manner to detect a tracking error constantly, a DC offset component of a tracking error signal will increase and a signal-to-noise ratio will be deteriorated due to influence caused by a signal at a pit position and this will cause an adverse affect on the tracking control and the spindle control based on the wobble signal and will also make it difficult to obtain ATip information (time information) contained in the wobble signal. Therefore, in a bottom period in which a pit is not formed, an attempt has been made to detect a tracking error in only a bottom period in which laser beam of a low power (bottom power) which is insufficient for forming a pit is projected (FIG. 2) and hold a tracking error signal of an immediately preceding value in a pit period.
For detecting a tracking error only in the bottom period, it is necessary, for obtaining a wobble signal of an adequate signal-to-noise ratio, to make as much time as possible in the bottom period available for detection of a tracking error signal. It is therefore desirable to make the whole bottom period available. However, as shown in FIG. 3B, the waveform of the reflected beam from a disk has a delayed trailing edge as compared with a laser diode output shown in FIG. 3A, and this causes leakage (a shaded portion in FIG. 3B) of a signal in the pit period to a bottom period. A frequency range of this leakage signal portion extends over a frequency range of the wobble signal as well as a frequency range of an EFM signal. Therefore, when a tracking error is detected throughout the entire bottom period, the signal-to-noise ratio of the wobble signal is deteriorated rather than improved.
Since the trailing edge of the waveform of the reflected beam from the disk assumes an exponential curve, it is necessary to determine a fall period .DELTA.t in such a manner that the trailing edge is decayed to such an extent that influence of the signal in the pit period is reduced and an adequate length of a tracking error signal detection period can be made available and to perform detection of a tracking error during the bottom period excluding this fall period .DELTA.t. In this case, the fall period .DELTA.t is determined at a length at which the best signal-to-noise ratio of the wobble signal is available by minimizing the influence of the signal in the pit period and maximizing the tracking error detection period.
The value of the fall period at thus determined is relatively small when the recording speed is a normal (one time) speed but, when the recording speed becomes higher than the normal speed, a relatively long fall period .DELTA.t is required as shown in FIG. 3C because it takes more time for the waveform of the reflected beam to decay completely. This results in shortening of the tracking error detection period with resulting deterioration of the signal-to-noise ratio of the wobble signal.
It is, therefore, an object of the invention to provide an optical disk recording device capable of preventing deterioration of signal-to-noise ratio of a wobble signal when the recording speed is changed.